1. Field of the Invention
The present invention relates to a magnetic thin film element making use of a giant magnetoresistive (GMR) effect, a memory element using the magnetic thin film element, and a method for recording and reproducing using the memory element.
2. Description of the Related Art
Although a magnetic thin film memory is a solid-state memory with no active part as is the case of a semiconductor memory, in the magnetic thin film memory, information is not lost even if a power supply is cut off, writing is enabled repeatedly up to an unlimited number of times, and there is no danger that the memory content may vanish with exposure to radiation, which are advantages in comparison with the semiconductor memory. In particular, recently, a thin film magnetic memory using the giant magnetoresistive (GMR) effect is receiving attention because of a larger output in comparison with a conventional thin film magnetic memory using an anisotropic magnetoresistive effect.
For example, in the Journal of the Japan Society of Applied Magnetics (Vol. 20, P.22, 1996), a solid-state memory is disclosed, in which a memory element is fabricated by depositing a plurality of times a structure including a hard magnetic layer (HM), a nonmagnetic layer (NM), a soft magnetic layer (SM), and a nonmagnetic layer (NM).
FIG. 1 is a schematic sectional view showing a structure of such a solid-state memory. In the drawing, numeral 1 represents a hard magnetic layer, numeral 2 represents a nonmagnetic layer, and numeral 3 represents a soft magnetic layer. In this solid-state memory, a sense line 4 is provided on both sides of the magnetic film, and a word line 5 is provided, and is isolated from the sense line 4 by an insulating layer 6. An electric current is applied to the word line 5 and the sense line 4, and information is written means of the magnetic field generated as a result.
Specifically, as shown in FIGS. 2A through 2D, by applying an electric current to the word line 5, a magnetic field is generated in a different direction in response to the direction of electric current represented by numeral 7. The magnetization of the hard magnetic layer 1 is reversed by the magnetic field to record a memory in a state of "0" or "1". In FIGS. 2A and 2C, the horizontal axis represents time T and the vertical axis represents electric current I. In FIGS. 2B and 2D, the same members as those in FIG. 1 are represented by the same numeral as in FIG. 1, and detailed descriptions will be omitted.
For example, by applying a positive current, as shown in FIG. 2A, to produce a rightward magnetic field, a memory state of "1" is recorded as shown in FIG. 2B. Also, by applying a negative current, as shown in FIG. 2C, to produce a leftward magnetic field, a memory state of "0" is recorded as shown in FIG. 2D.
In order to read information, as shown in FIGS. 3A through 3E, an electric current 7 that is smaller than the recording current is applied to the word line 5 to reverse the magnetization of the soft magnetic layer 3 only, and a resulting change in resistance is detected.
In FIG. 3A, the horizontal axis represents time T and the vertical axis represents electric current I. Also, in FIGS. 3B through 3E, the same members as those in FIG. 1 are represented by the same numeral as in FIG. 1, and detailed descriptions will be omitted.
When the giant magnetoresistive effect is used, resistance varies depending on whether the magnetizations of the soft magnetic layer SM and the hard magnetic layer HM are parallel or antiparallel. Thus, a memory in a state of "1" can be discriminated from a memory in a state of "0" in response to the change in resistance. For example, as shown in FIG. 3A, when a current is applied as a positive pulse and then a negative pulse, the magnetization of the soft magnetic layer 3 changes from rightward to leftward, and with respect to the memory in a state of "1", a small resistance as shown in FIG. 3B changes is replaced by a large resistance as shown in FIG. 3C. On the other hand, with respect to the memory in a state of "0", a large resistance as shown in FIG. 3D changes is replaced by a small resistance as shown in FIG. 3E. By detecting the change in resistance as described above, information recorded in the hard magnetic layer HM can be read regardless of the magnetization condition of the soft magnetic layer SM after recording.
In the conventional magnetic thin-film memory having the structure described above, however, as the area of the memory cell decreases, a demagnetizing field (self-demagnetizing field) generated in the magnetic layer increases so as to not be negligible, and the magnetization direction of the magnetic layer that stores records is no longer fixed in one direction, resulting in instability. Therefore, in the conventional magnetic thin-film memory, the refinement (reduction in size ) of a one-bit cell and the stable storage of information are incompatible, and high integration is impossible.